Sadly, even if an 84 yr old 'has no mental issues', it is not unusual for someone to suddenly be confused or have a sudden drop in competency.
My mom , when she was in assisted living at 86 and 87, used to get UTIs, and suddenly she would be acting like she had dementia, and she would go wandering alone, looking for who knows what. She was picked up by police both times it happened and we had to help her move to Memory Care instead of her living independently.
Someone in their mid 80's may be fine 80% of the time, but there can be sudden mental glitches that pop up sometimes.
But it does sound like someone harmed her, so it was probably someone who knew she was alone at night...